Today's integrated circuits include a vast number of devices. Some advanced devices may benefit in several ways, for instance, by improved performance if they can be fabricated to include more than one type of semiconductor material. The mainstay material of microelectronics is silicon (Si), or more broadly, Si based Group IV semiconductor materials. One such Si based material of importance for microelectronics is the silicon-germanium (SiGe) alloy. For many applications, often involving communication and optical capabilities, Group III-Group V compound semiconductors bring useful properties. Typical representatives of Group III-Group V compound semiconductors, or III-V semiconductors for brevity, are GaAs, GaAlAs, InAs, InP, and many others. Combining Group IV and III-V semiconductors otters many advantages, and has a long history in the art.
The usual, and useful, way of combining Group IV and III-V semiconductors into common structures is by epitaxially depositing one of the materials onto the other. Often one is interested in depositing a Group IV semiconductor epitaxial layer onto the surface of a III-V compound semiconductor. When two different semiconductors, or more generally two different materials, have a common interface it is referred to as a heterojunction.
Ge, Si, SiGe, representative Group IV semiconductors, have been traditionally deposited on GaAs, a representative III-V semiconductor, by molecular beam epitaxy (MBE). The GaAs surface is typically heated to a temperature exceeding 575° C. in an ultra-high vacuum environment of <10−9 torr in order to desorb the native gallium arsenide oxide before Ge, Si or SiGe growth can be initiated. However, at high temperatures in vacuum, arsenic desorbs from the GaAs lattice, leaving excess Ga which tends to form droplets, and subsequently, a very rough 3D surface. High overpressures of arsenic containing species have been used to minimize arsenic desorption, but AsH3 is toxic and difficult to handle safely (Golfarb et al. J. APPL. PHYS. 93, 3057, 2003). Similar techniques are employed in magnetron sputtering of Ge/GaAs superlattices by Rosendo et al. J. Appl. Phys. 89, 3209, 2001, where following a wet clean a high vacuum surface oxide degas was necessary prior to Ge growth. However, deposition of Ge on GaAs superlattices at elevated temperatures, such as 580° C., usually resulted in Ge interdiffusion into the GaAs layer. Additionally, significant interdiffusion of Ge and GaAs is observed even at more moderate MBE growth temperatures of 430° C.
The phases of oxides on GaAs may be AS2O3 and Ga2O3. The thermodynamics and kinetics indicate that a III-V semiconductor surface, such as a GaAs surface, exposed to air will form a native oxide consisting of gallium oxide and free arsenic.
To counteract the III-V semiconductor surface native oxidation process, several passivants have been proposed through the years. However, none to date has yielded fully satisfactory results.